I've built a few ebikes over the last few years , with my last one being a 3kW cyclone middrive one. This was stolen recently so am looking for the next project... was wondering how easy/practical it would be to convert a 125cc motorbike to electric? In the UK a decent suspension mountain bike is circa £1000, and if you fit a strong motor to it, it may need upgrading to cope with the stresses imposed (motorbike rims, tyres etc).

However a non-running motorbike can be had for £300 and will already have good brakes, wheels, tyres etc? Who has converted a low powered motorbike and got it on the road legally?

Depends. Are you interested in running a hub or mid drive setup? The shape of the frame will determine what kind of motor is easiest to install. Hub motors are easier to setup but require higher amp batteries to deliver similar performance to mid drive due to the lack of gearing. Mid drive can utilize gearing but bikes chains and derailleurs kind of suck under high power, talking 1500w+. The cyclone is great but it snaps chains left and right. Let me know what your plan is.

Scooby, I am interested doing something similar. I have found a little more info on elmoto.net since their focus seems to be motorcycles in particular. My opinion is converting a 125cc bike or similar size would be much more durable and potentially safer but the battery required to power a larger motor may end up adding some weight to the build. Depending on what you wanted to do with the bike, commute or make short trips, the range on an acceptable battery might not be up to the task.

Someone on here tried to fit an electric motor to a full sized dirt bike and had some issues burning out the motor. Another guy took some forklift parts to put together a bike but that is a pretty 1 off project in my opinion. From the few I have seen done well, I think you need 5-8 kW motor and you may need to find the right gearing to make full use of the rpms.

If you do find a good example or get started, I would love to follow the thread.

I've seen a few UK builds that got plates. One was a Honda (Mt 5?) as a moped 45 km/h, it is here on the forum somewhere. It seems the process of getting converted bikes road legal in UK is easier then in many other EU countries.

If you get your converted bike registered as A1 you will have no speed limitations and can ride highways as well.

Come to think of it, there was also a white vector bike in the UK with plates. Street legal.
And a scooter conversion. Probably the easiest way is to ask one of blokes that already been down that road getting bikes road legals.

The thread is here if you want to check up on the details.
The 50cc bikes of the "slowped" era are a lot lighter than the 125cc stuff, which is the main reason I chose this one.

The main advice I can give is to decide if you want a moped class or a motorcycle class - the moped class is limited to 4KW and 30mph.
Changing the class of a moped to a motorcycle with the DVLA is probably just as much hassle as getting them to swap the power from petrol to electric, so best if you can get the right donor to start with.
I agonised long and hard over 50cc or 125cc... in the end I went with light weight.

It did not actually cost any money to get the registration changed over from petrol powered to electric powered with the DVLA (Driver and Vehicle Licensing Agency) here in the UK. Just a matter of filling in forms correctly. If you check out my build thread it details the steps I had to go through. This only works if it is considered to be an engine swap, if it gets considered as modified then it may need to go through MSVA (Motorcycle Single Vehicle Approval) which is a lot more involved and does cost money.

So it pays to read through the MSVA regulations (here if you are really bored) and try and make sure whatever you come up with does not mean it will require an MSVA inspection. I seem to remember that the requirements started in 2003 after which all vehicles sold had to have a COC (certificate of Conformity) so it is definitely better to use a donor registered before that date.

Ideally for a car conversion you would stay away from hub motor due to unsprung weight and the negative impact it has. Due to weight of of the car you would have to use rather large and heavy hub motors.

An electric w210 that would be really cool. I'll hope to see that build thread.